In general, efficiency is considered to be one of the most important figures of merit for a radio frequency (RF) device such as a power amplifier. An efficient power amplifier requires efficient conversion from a supplied direct current (DC) power into a transmitted RF power in order to maximize battery lifetime of e.g. a mobile terminal or to minimize power consumption and thus operating expenditures (OPEX) and heat generation of a radio base station.
In addition, switching mode power amplifiers such as class-E, class-F and inverse class-F power amplifiers operate efficiently by minimizing the overlapping transition region between consecutive voltage and current pulses. It is well known that with harmonic load impedances terminating properly, the voltage and current harmonic components of the RF output signal can be controlled and thus voltage and current pulse-shaping is achieved.
For example, an ideal class-F amplifier requires a low impedance (ideally a short-circuit termination) relative the impedance at a fundamental frequency, denoted here f0, for all even harmonics and a high impedance (ideally an open-circuit termination) relative the impedance at f0 for all odd harmonics to be presented, in order to suppress even harmonic voltage components and odd harmonic current components, so that consecutive non-overlapping square-wave voltage pulses and half sinusoidal current pulses are achieved. By a harmonic component is meant a multiple (odd or even) of the fundamental frequency f0.
In practical applications, the class-F amplifier is thus provided with a harmonic control circuit (HCC) on the output port of the amplifier which is approximately short-circuited for at least the first even harmonic, 2f0 and approximately open-circuited for at least the first odd harmonic, 3f0.
Harmonic control circuits can for example be implemented on a separate printed circuit board (PCB) provided outside an active device package or an active device bare-die. Another way of HCC implementation is on-chip, either with bondwires connecting capacitors (e.g. metal insulator metal (MIM) capacitors) or integrated passive devices (IPD) or monolithically integrated on monolithic microwave integrated circuits (MMICs). The passives can be either on the same semiconductor substrate as the active device or on a separate substrate.
In U.S. patent No. 2007/0057731 A1 there is disclosed a HCC for a RF power amplifier comprising an on-chip transistor formed on a semiconductor substrate and an on-chip harmonic termination formed on the same substrate. In this prior art, two resonator cells are provided wherein on-chip harmonic termination is configured to provide a short-circuit termination for even-harmonics of the RF output signal and to provide an open-circuit termination for odd harmonics of the RF output signal. The HCC is here implemented with bondwires and MIM capacitors, and is provided in cascade with an output matching network.
HCCs with more than one resonator cell can also be used to control subparts of the same frequency band in order to increase bandwidth. This is disclosed in U.S. Pat. No. 7,176,769 B1.
It is further known that a transmission line of a predetermined physical length being terminated with either a short-circuit or with an open-circuit in one end, provides an alternating short-circuit termination or an open-circuit termination in the other end with a periodicity of twice the frequency at which the electrical length of the transmission line corresponds to a quarter of a wavelength. It is also known that if such a terminated transmission line is connected in shunt to a mainline, a so called quarter-wave stub is realized, providing an alternating short-circuit to the mainline junction point or being invisible to the main-line with the same periodicity. Furthermore, if the junction point of such a quarter-wave stub is offset an arbitrary electrical length from a first mainline terminal, any reflection angle between an open-circuit (i.e. reflection angle 0°) and a short-circuit (i.e. reflection angle 180°) can be obtained.
Consequently, by shunting a mainline with one or more stubs of equal or of different lengths at predetermined offset positions relative a first mainline terminal, a single harmonic or a multiple order HCC is achieved. One example of an HCC implemented in accordance with the above described prior art is described in US patent document No. 2008/0191801. In this prior art, the HCC is implemented with microstrip on PCB and is provided in cascade with an output matching network.
A drawback with the HCC disclosed in this prior art, is the large board area being occupied by the required lengths of series and shunt transmission lines. Another drawback relates to the cost, loss and tolerance when using short-circuited stubs since the use of short-circuited stubs generally require means for DC-blocking. As an example, in order not to short-circuit the DC-supply of the active device, typically a DC-blocking capacitor is required.
Another disadvantage is the inherent periodicity of the quarter-wave stubs, which makes it hard to independently control multiple harmonics. Yet another disadvantage is the undesired reactive component at other frequencies than the controlled harmonic(s). As an example a 3f0 quarter wave stub does not provide a high enough fundamental impedance in order to being invisible at f0. Consequently it is hard to independently control multiple harmonics and fundamental matching in a matching network incorporating such a prior-art HCC.
For large power devices, which usually include a number of small amplifier cells on a common or on separate dies, the cells are paralleled to a common relatively wide output lead by a corporate feed network. Increasing the number of paralleled device outputs lower the required fundamental impedance for optimum efficiency and output power. Wide low impedance output lines on PCB are therefore typically required both to accommodate the wide device leads and to match to the relatively low optimum fundamental impedance.
It should be noted that it is known that a shunt stub having a significantly higher impedance than the mainline shunted becomes more narrow-band compared to a lower impedance stub i.e. the frequency range over which essentially a short-circuit termination is presented is reduced. It is also well known that such a relatively high impedance stub provides a less effective short-circuit termination at the resonant frequency i.e. the desired zero ohms short-circuit termination is approximated with a higher impedance.
Another consequence of the relatively wide low impedance output line required for high power devices is that the corporate feed network together with the output matching network form a distributed combiner network and for a high power device the harmonic termination presented to each cell can vary due to varying electrical length to the harmonic short-circuit termination, with reduced efficiency as a result. Consequently a prior art HCC is hard to combine with a low impedance fundamental matching network required for large high power devices.
It should be noted that another class of prior art HCCs on PCB not incorporating quarter-wave stubs are known as electromagnetic band-gap (EBG) structures, and defected ground structures (DGS). EBGs are relatively large periodic structures typically utilized for harmonic suppression i.e. filtering. Due to the distributed nature of EBGs these are not suitable for controlled termination angles. DGS on the other hand are smaller than EBG but require etched ground plane patterns, which are not suitable for implementation with microstrip in multi-layer boards and are also less compatible with mounting on PCB to metal heatsinks.